Recent studies in our laboratory have established for the first time that the forearm area of the primate motor cortex contains: (i) a large population of very small pyramidal tract (PT) cells, the majority of which send their axons to the contralateral cord; and (ii) a separate population of small corticorubral neurons, that exert excitatory effects upon rubrospinal tract cells. Together, these two populations account for sixty-five percent of the efferent cells that lie within this region of cortex, and that send their axons to the brain stem or spinal cord. It is quite clear from careful reviews of the literature, however, that most of our concepts about the specific functions of the primate motor cortex are in fact based primarily upon studies of its large, rapidly conducting PT cells. Additional studies of the functions of these small cell systems are therefore badly needed. For a number of reasons, it is our belief that these small neurons may participate strongly in generating the steady muscular forces that underlie the voluntary support of a fixed limb posture or muscle load, whereas the fast PT system may function largely to generate the transient forces that underlie rapid, skilled movements. We are therefore proposing three sets of experiments that are designed to provide new information about the voluntary 'transient' and 'tonic' motor functions of the primate's slow pyramidal and corticorubrospinal pathways. In the first set, microelectrode recording techniques will be used to observe and compare the behavior of large samples of these small cells with that of fast PT neurons, in monkeys that have been trained to generate both transient and maintained, steady outputs in forelimb muscle force. In the second set, additional information will be obtained about the functions of the rubrospinal system, by recording the activity of cells within the arm area of the red nucleus in animals that have been trained to perform precisely the same set of forearm motor tasks. Finally, a careful, quantitative electromyographic study will also be carried out, in order to determine the specific motor deficits that are produced in the performance of these same tasks by surgical interruption of pyramidal and corticorubrospinal pathways.